Control Device with Terminal 15 - Holding Circuit

ABSTRACT

A terminal state of a terminal  15  of a vehicle is enabled to be maintained for a minimum period of time even after a drop in the voltage supply. At the same time other loads are to have no effect on the holding time. A control device for controls access to a vehicle and has a voltage source, a voltage supply terminal to which a supply voltage can be applied, and a holding circuit to which the voltage source and the voltage supply terminal are connected in a common circuit point for the purpose of holding an On state for as long as the voltage at the voltage source does not fall below a predetermined value. A switching element that is connected into the circuit between the voltage supply terminal and the common circuit point serves to separate the common circuit point from the supply voltage. The holding time is thus decoupled from the VCC currents drawn by other loads.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of Germanapplication DE 10 2006 046 032.4, filed Sep. 28, 2006; the priorapplication is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of Invention

The present invention relates to a control device for controlling accessto a vehicle based on a state of a terminal 15 of a vehicle electronicssystem. The control device contains a voltage source, at least onevoltage supply terminal to which a supply voltage can be applied and aholding circuit to which the voltage source and the at least one voltagesupply terminal are connected in a common circuit point for the purposeof holding an On state for as long as the voltage source does not fallbelow a predetermined value.

In certain situations it is necessary for the hardware terminal stateKL15 (ignition) to be maintained for a certain time even if the batteryvoltage dips. For example, the engine of the vehicle is not to beswitched off if a battery voltage drop occurs temporarily.

In control devices in the vehicle, a dipping of the battery voltagebelow 5.5 V is interpreted for example as a battery voltage drop. Afterthe voltage drop has been detected, the aim is that it shall be possiblefor the hardware terminal state KL15 to be maintained for a minimumholding time th_min. At the battery voltage Ub<5.5 V, the control deviceand the microcontroller are in a hardware RESET state. A RESET alsotakes place at a voltage supply in the range of VCC=0 . . . VCC_min(minimum voltage supply).

FIG. 1 shows a circuit diagram of a holding circuit of a control deviceof the aforesaid kind, the holding circuit being implemented by discretecomponents. The current taken from the terminal KL15_ON is drawn fromthe capacitor C1. The latter is connected via a diode D1 to a terminalKL30L (corresponds to battery voltage source, e.g. 0 . . . 28 V). Viathis path the capacitor C1 can be fed with battery voltage Ub. Inparallel herewith the capacitor C1 is connected via a diode D2 to aterminal VCC (voltage source for microcontroller electronics, logic andauxiliary electronics). Via the terminal VCC the capacitor C1 issupplied with logic supply voltage of 5 V. The capacitor C1 thus has aredundant voltage supply.

The current from the capacitor C1 to the terminal KL15_ON is controlledvia a PNP transistor T1. For this purpose the capacitor C1 or thecathodes of the diodes D1 and D2 is/are connected to an emitter of thetransistor T1 and a terminal KL15_ON is connected to a collector of thePNP transistor T1.

The holding circuit can be controlled by an external SET or CLR command.Toward that end a base of the transistor T1 is connected to the emitterof the transistor T1 via a resistor R1. In addition, the base of thetransistor T1 is connected to the collector of an NPN transistor T2 viaa resistor R2. The emitter of the NPN transistor T2 is connected toground. A voltage divider R3, R4 divides down the voltage of an inputterminal KE for the base of the transistor. SET and CLR commands in theform of pulses can be applied to the terminal KE by a microcontroller.For the purpose of transferring the potential the terminal KL15_ON isalso connected to the input terminal KE via a resistor R5.

In order to hold the terminal state KL15_ON, a SET command in the formof a positive pulse is applied to an input terminal KE with the resultthat the potential there increases temporarily (high level). Thetransistor T2 thereupon conducts and consequently so too does thetransistor T1. The terminal KL15_ON is raised to the high potential ofthe capacitor C1, causing the transistor T2 to continue to conduct. Thefeedback via R5 produces the self-holding of the circuit. The drawncurrent flows from the capacitor C1 through the transistor T1 to theterminal KL15_ON.

The transistor T2 and hence also the transistor T1 can be turned off bya CLR command (clear) which is implemented by a brief pulse to ground(low level). This causes the current flow from the buffer capacitor C1to be broken off.

The following applies to a battery voltage drop. If the self-holding isnot actively terminated by a CLR command, the voltage at the buffercapacitor C1 and consequently also the base-collector voltage Ube of thetransistor T2 fall below a predetermined level, with the result that thetransistor T2 and also the transistor T1 turn off (non-conducting).

The supply voltage VCC is 5 V in normal operation. However, if it fallsbelow 4.8 V and the battery voltage Ub at the terminal KL30L is lessthan 5.5 V, a RESET of the control device takes place.

A disadvantage with the circuit shown in FIG. 1 is that other loadsconnected to the VCC network have a direct influence on the holding timeth_min, for this is dependent on the current drawn from the buffercapacitor C1. A further disadvantage is that the dielectric strength ofthe buffer capacitor C1 must also be dimensioned for the vehicleelectrical system.

BRIEF SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a control devicewith terminal 15—holding circuit that overcomes the above-mentioneddisadvantages of the prior art devices of this general type, whoseminimum holding time is independent of other loads and the state of thebattery voltage.

The object is achieved according to the invention by a control devicefor controlling access to a vehicle based on a state of the terminal 15of a vehicle electronics system. The control device has a voltagesource, at least one voltage supply terminal to which a supply voltagecan be applied, and a holding circuit to which the voltage source andthe at least one voltage supply terminal are connected in a commoncircuit point for the purpose of holding an On State of the terminal 15for as long as the voltage at the voltage source does not fall below apredetermined value. A switching element is inserted in a circuitbetween the voltage supply terminal and the common circuit point so thatthe common circuit point can be separated from the supply voltage.

This advantageously achieves an independence of the minimum holding timefrom the VCC network. The holding circuit is also independent of thebattery voltage, since the voltage supply of the control device isprovided by the VCC network during normal operation.

The voltage source is preferably a buffer capacitor that is fed from theVCC network.

A particular advantage is also that the buffer capacitor can bedimensioned for a nominal voltage between 4 V and 6 V, in particular for5 V, if the supply voltage of the control device corresponds to justthis nominal voltage. This results in cost benefits, since capacitorsfor 35 V vehicle electrical systems are considerably more expensive.

The voltage source VCC and VCC_2 can, however, also be provided by astorage battery or other appropriate devices, although these are usuallymore expensive than simple buffer capacitors.

The switching element which is connected into the circuit between thevoltage supply terminal and the common circuit point of the controldevice according to the invention can be a MOSFET which is controlled byan externally provided RESET signal. The RESET signal thus provides aclearly defined time as of which the holding time of the holding circuitruns (trigger signal).

The holding circuit in the control device according to the invention canbe implemented for example by a D flip-flop. A D flip-flop of this kindis available as a standard component.

Alternatively the holding circuit can also be built using discretecomponents exclusively, e.g. two bipolar transistors and a plurality ofresistors. A circuit of this kind can be individually configured.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a control device with terminal 15—holding circuit, it is neverthelessnot intended to be limited to the details shown, since variousmodifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a circuit diagram of a flip-flop holding circuit for terminala KL15 according to the prior art;

FIG. 2 is a circuit diagram for providing a special supply voltageaccording to the invention; and

FIG. 3 is a circuit diagram of a D flip-flop for holding the state ofthe terminal KL15 with the supply voltage provided according to FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown a hardware terminalstate KL15_ON (ignition) that is maintained for a minimum holding timeth_min, with a controlling microcontroller not necessarily having to beactive, as indicated by a level RESET=Low. This state is assumed forexample if the voltage supply VCC drops below a predefined thresholdvalue.

According to the invention a new supply voltage VCC_2 is generated forthis purpose in a control device for controlling access to a vehicleaccording to the circuit diagram shown in FIG. 2. The supply voltageVCC_2 corresponds to the voltage at the buffer capacitor C2 for theholding circuit.

The buffer capacitor C2, and consequently also a point P at which thesupply voltage VCC_2 is tapped, is connected to the terminal VCC via thedrain-source section by use of a p-channel MOSFET M1. A diode D1 whoseanode is directed to the terminal VCC runs parallel to drain and source.A gate of the MOSFET M1 is connected to ground via a drain-sourcesection of an n-channel MOSFET M2. A diode D2 with its anode directed toground is in turn disposed in parallel with the drain-source section. Agate of the MOSFET M2 is coupled to a RESET terminal via a resistor R6.

Thus, a new supply voltage VCC_2 for a KL15 flip-flop which is shown inFIG. 3 is generated from the supply voltage VCC for logic andmicrocontroller, which supply voltage VCC is typically 5 V.

A terminal 15 signal KL15_ON is generated according to the example ofFIG. 3 by a D flip-flop DF. The D flip-flop DF has a first supply inputGND which is connected to ground. A second supply input VCCE isconnected to the new voltage supply terminal VCC_2 and protected againstground by a capacitor C3. The input PRE of the D flip-flop DF is alsoconnected to the new supply voltage terminal VCC_2 via a resistor R7. Inaddition the input PRE is also connected to a terminal K1 by which a SETcommand can be input.

The two inputs D and CLK of the D flip-flop are connected to ground. Theinput CLEAR of the D flip-flop is connected to a terminal K2 via which aCLEAR command, i.e. a low level, can be input. A positive output QPOS ofthe D flip-flop is connected to the terminal KL15_ON. By this circuitthe output terminal KL15_ON is set to 1 (high level) when the commandSET is present (SET=1). Conversely, the output terminal KL15_ON ispulled to the low level 0 when the CLR command is sent to the inputCLEAR of the D flip-flop by a pulse to ground.

In the normal operating state, when sufficient voltage is present andthe ignition is switched on, the terminal state is KL15_ON=1 (i.e. athigh level). The input levels of the terminals K1 and K2 are also high,i.e. SET=CLR=1. The RESET terminal of the circuit of FIG. 1 is thenlikewise at high level, i.e. RESET=1. In this case the new voltage isVCC_(—)2˜VCC-0.1V.

If the voltage drops below a critical value, a voltage monitoringcircuit supplies the signal RESET=0. In this case the terminal stateKL15_ON=1 must be maintained for a minimum holding time th_min. Thisstate can be maintained for a certain period of time by the charge stateof the buffer capacitor C2, since the buffer capacitor C2 provides thenecessary energy for the D flip-flop DF and the signal KL15_ON.

The holding circuit is therefore activated according to FIG. 1 by acommand RESET=0. The activation signal simultaneously serves as atrigger signal, for it is used to define a definite time as of which thestate of the terminal 15 is held.

The holding circuit is decoupled from the VCC terminal by the MOSFET M1.This is important in particular during the transition from a normal modeof operation, in which the supply voltage is sufficiently high and noRESET command is present (RESET=1), to a RESET state (RESET=0) as wellas in the RESET state itself (RESET=0). This is because the holding timeis then not dependent on other loads which are connected to the VCCsupply voltage. Only the holding circuit itself is connected to thecapacitor C2. As a result, given a smaller capacitance of the buffercapacitor C4, an identical holding time th_min can be achieved as inprior art solutions. The effect of the VCC current drawn by other loadsthus has no impact on the holding time and more particularly on thevoltage VCC_2.

Because no redundancy feeding of the capacitor C2 by the battery voltageUb is provided, the holding time is thus automatically decoupled fromthe battery state Ub. The decoupling provides the further advantage thatthe buffer capacitor C4 must be dimensioned only for the nominalvoltage, e.g. VCC=5 V, but not for a higher vehicle electrical systemvoltage, e.g. U_(b)=35 V. It is also not necessary for VCC>VCC_min andfor an extremely small VCC drawn current to be flowing in order toguarantee the minimum holding time th_min, as is necessary with thecircuit shown in FIG. 1.

1. A control device for controlling access to a vehicle based on a stateof a terminal 15 of a vehicle electronics system, the control devicecomprising: a voltage source; at least one voltage supply terminal towhich a supply voltage can be applied; a holding circuit coupled to saidvoltage source at a common circuit point for holding an On state of theterminal 15 for as long as a voltage at said voltage source does notfall below a predetermined value; and a switching element connected intoa circuit between said voltage supply terminal and said common circuitpoint so that said common circuit point is separated from the supplyvoltage.
 2. The control device according to claim 1, wherein saidvoltage source is a buffer capacitor.
 3. The control device according toclaim 2, wherein said buffer capacitor is dimensioned for a nominalvoltage between 4 V and 6 V.
 4. The control device according to claim 1,wherein said voltage source is a storage battery.
 5. The control deviceaccording to claim 1, wherein said switching element is a MOSFETcontrolled by an externally provided RESET signal.
 6. The control deviceaccording to claim 1, wherein said holding circuit is a D flip-flop. 7.The control device according to claim 1, wherein said holding circuit isbuilt exclusively using two bipolar transistors and resistors.
 8. Thecontrol device according to claim 3, wherein the nominal voltage is 5 V.